This invention relates to a digital frequency meter for use in radiation monitors which are installed in a nuclear power station, etc.
A prior-art apparatus of this type has been as illustrated in FIGS. 1A and 1B. In FIG. 1A, numeral 1 designates a present counter, numeral 2 a timer, and numeral 3 a computer.
Pulses to be measured are applied to the present counter 1, and are counted to a present number (denoted by N). The timer 2 measures the period of time (denoted by T) taken for the preset counter 1 to count the N pulses to-be-measured.
The quantities N and T are inputted to the computer 3, and the frequency of the pulses to-be-measured is obtained via the following calculation: ##EQU1##
FIG. 1B shows an example of the operating waveforms of the apparatus shown in FIG. 1A. The computer 3 generates a measurement start pulse P.sub.START at a time t.sub.s1. This pulse P.sub.START resets the preset counter 1 and the timer 2 into the initial statuses thereof. Thenceforth, the preset counter 1 counts the pulses to-be-measured, and the timer 2 counts reference time pulses (not shown), to increase their count values. When the count content of the preset counter 1 has reached the preset value N at time t.sub.E1, this preset counter generates a measurement end pulses P.sub.END. This pulse P.sub.END stops the counting operation of the timer 2, and informs the computer 3 of the end of the measurement. Upon receiving the pulse P.sub.END, the computer 3 executes a series of operations including the loading of the count value T.sub.1 of the timer 2, the calculation of the frequency, the updating of the frequency output, and the generation of the measurement start pulse P.sub.START at a time t.sub.s2.
By repeating the above operations, the frequency of the pulses to-be-measured is measured.
Since the prior-art digital frequency meter is constructed and operated as described above, it requires the counter for counting the pulses to-be-measured and the timer having a dynamic range equivalent, at least, to that of the rate of the pulses to-be-measured and the division is necessary within the computer. Another disadvantage is that the frequency which is outputted every cycle of sampling changes discontinuously. Particularly in case of a low frequency, the sampling cycle becomes long, and the discontinuous change of the frequency to be outputted every sampling cycle is unfavorable for the radiation monitor for which the monitoring of the slow variation of a radiation level is also important.